Precious metals are used as protective films on surfaces for a variety of reasons. In the jewelry trade, it is used to improve the appearance of an article as in gold plated jewelry. In other applications, it is used to protect against corrosion of metals and other surface materials. In the electrical arts protective films made of precious metals are used as conduction paths in electrical circuits and as contact surfaces in devices with electrical contacts. Gold is used extensively in these applications with great success. However, the increased price of gold makes it attractive to look at other precious metals as protective films on various surfaces.
Palladium and palladium alloys are used extensively in a variety of industrial applications. Typical examples are the jewelry trade where such films are used to protect surfaces against corrosion and to improve appearance, in the electrical arts in various electrical devices and electronic circuits and in the optical field for various types of optical devices.
Because of chemical inertness and reasonable hardness, palladium is especially attractive as an electrical contact material in electrical connectors, relay contacts, switches, etc. Various palladium alloys such as palladium-silver, palladium-nickel, and palladium-copper are also useful for the same applications. Indeed, because of the increasing cost of gold, palladium and palladium alloys become more and more attractive economically as a contact material, surface material, and in other applications. In many applications where gold is now used, it is often economically attractive to use palladium, provided an inexpensive and efficient method of plating ductile and adherent palladium is available.
Highly desirable is a process for plating palladium from an aqueous solution which is rapid and yields palladium and palladium-alloy films which are ductile, adherent and free from hydrogen. Further, it is desirable to have a palladium electroplating process which does not require subsequent heat treatment to remove hydrogen, improve ductility or adherence. In many applications, it is desirable that the palladium plating bath not chemically attack the surface being plated so that the bath remains uncontaminated during the plating process. Palladium plating processes have been described in a number of references including U.S. Pat. No. 1,970,950, issued to E. M. Wise on Aug. 21, 1934; U.S. Pat. No. 1,993,623, issued to A. R. Raper on Mar. 5, 1935; and U.S. Pat. No. 3,290,234, issued to E. A. Parker et al on Dec. 6, 1966. Ethylenediamine has been used in a palladium alloy plating procedure (U.S.S.R. Pat. No. 519,497 issued June 30, 1976); (C. A. 85: 113802m) and it was known to the inventors that ethylenediamine is useful in palladium electroplating in the following composition bath: 28 gm/ 1 PdCl.sub.2, 140 gm/1 Na.sub.2 SO.sub.4 and sufficient ethylenediamine to dissolve the PdCl.sub.2. The bath is used at room temperature, the current density is 20 mA/cm.sup.2 and the pH between 11 and 12.